[llvm-dev] Allowing PRs on GitHub for some subprojects

Thu Mar 5 08:30:14 PST 2020

> On Mar 4, 2020, at 20:15, Mehdi AMINI <joker.eph at gmail.com> wrote:
> 
> 
> 
> On Wed, Mar 4, 2020 at 9:42 AM Louis Dionne <ldionne at apple.com <mailto:ldionne at apple.com>> wrote:
> 
> 
>> On Mar 4, 2020, at 12:13, Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com>> wrote:
>> 
>> 
>> 
>> On Wed, Mar 4, 2020 at 8:14 AM Louis Dionne <ldionne at apple.com <mailto:ldionne at apple.com>> wrote:
>> Mehdi, Chris & others,
>> 
>> I guess I did not express the main reasons for wanting to switch over very well in my original message. 
>> 
>> You original message was about “ commit attribution”, but now it is all about testing?
> 
> It was about allowing GitHub PRs for libc++, and one of the reasons I cited was commit attribution.
> 
>> 
>> Instead of jumping to a solution (pull-request) why not expressing the actual problem (lack of pre merge testing) and discuss it and explore all the possible solutions? 
>> I think the discussion would be much more productive if we take it from first principles here.
> 
> That's really what I've tried doing below. Quoting myself:
> 
> 	> And if the solution is that Harbormaster suddenly becomes usable without an unreasonable time investment from me, then I'm fine with that too. I'm not looking to switch to GitHub PRs for the sake of it, I'm looking to solve problems that are harming libc++ in the current system.
> 
> Maybe I should have stated that earlier on.
> 
>> 
>> 
>> Like Christian talked about, for me it's all about pre-commit testing. I believe pre-commit testing is a widely shared desire among this community. However, how badly it is missed depends on sub-projects, because they have different realities. For example, in libc++:
>> 
>> 1. We have a lot of first-time contributors, which means that the maintainers end up shepherding many contributions. In particular, this often means fixing small breakage following their changes, which can be difficult for them because they can't reproduce the failures locally, and they might not even know where to look. While these contributors can submit valuable improvements and bug fixes, we can't expect them to fix every last platform that we support in the current state of things -- it's hard, it's boring, and it's stressful.
>> 
>> 2. Our testing matrix is very large, and interactions between different configurations (usually #ifs/#elses) is very subtle. This means the rate of mistake-on-first-try is, I think, higher in libc++ than in most other LLVM projects. Even with careful review, I find that a large percentage of changes end up breaking something somewhere, and I have to fix it (usually quickly enough to avoid reverting).
>> 
>> As a result, the lack of pre-commit testing is actively harming the health of libc++ as a project. It might be true for other projects as well, but I can only speak for libc++ because that's where I have first hand experience. 
>> 
>> What changed recently that makes this suddenly critical compared to the previous years?
> 
> At the risk of oversharing, this has been my biggest source of pain and frustration since I've became a libc++ maintainer. It's always been a problem since I started about 2 years ago. I can still feel the hurt of fixing the ripples of landing aligned allocation in libc++ in 2018, which lasted for weeks and weeks (on and off).
> 
> Last week, I spent about 4 days fixing the ripples of a high-quality contribution that happened to break some bot configurations (and some botless ones too). That was both very stressful and frustrating, and I swore to myself this was going to be the last week like that.
> 
>> 
>> Unfortunately, we currently don't have a good way of doing pre-commit testing on Phabricator AFAICT
>> 
>> 
>> I thought we do now? I got a bunch of libcxx failing on my revision a few weeks ago.
> 
> My understanding is that Christian was the one to setup the pre-merge testing we have currently (thanks!). And he says:
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>     > This is soooooo much easier to set up and maintain than the Phabricator integration
> 
> Of course it is easier to do on GitHub, but the integration on Phabricator can also a one-time cost for the project, which is already kind of on the way with https://github.com/google/llvm-premerge-checks <https://github.com/google/llvm-premerge-checks> ; then it is a matter of making it easier to add your own bot the pre-merge and increase the coverage.

You seem to be dismissing the cost of setting up bots as being something that has to be paid once. In my experience, however, this is actually a recurring cost because these bots need maintenance. And whenever we add a new bot, it's also a problem if we are confronted to this complexity again.

But like I said before, I'm only here to solve my (libc++) problems. And right now, my problem is that I *literally can't* figure out how to setup a pre-merge bot on Phabricator. Do you know how to do that? In all seriousness, can you show me how to set up even a simple bot that would run the most basic libc++ configuration? I'm sure I would be able (and certainly willing) to extrapolate from there and set up the other missing configurations without your help.

But if you can't help me, then who can? Honestly, I think we're all talking out of our hats and the only person who truly has gone through the process is Christian -- and you've seen his email.

Cheers,
Louis

>  
> Furthermore, what libc++ needs in this area is not necessarily the same as the rest of LLVM. I don't know what pre-merge bots are setup right now, and it's certainly better than nothing, but we have a lot of test configurations that just don't really make sense for a toolchain:
> - test suite in c++03/c++11/c++14/etc
> - exceptions on/off
> - with per-TU guarantee enabled/disabled
> - all the macOS back-deployment targets against system dylibs
> - etc.
> 
> I just see this as different configurations / builders that you have to provide if you care about, just like our current post-merge buildbot infrastructure actually: http://lab.llvm.org:8011/buildslaves <http://lab.llvm.org:8011/buildslaves>
> For example MLIR would want to run test on machines with GPUs (to test our CUDA and Vulkan paths), which are of no interest to most of the other projects, yet that seems fairly orthogonal to the mechanism to register a new bots / config with the system.
> 
>  
> 
> I'm not expecting anyone else to set these up cause it doesn't make sense. I need to be empowered to do it myself, and right now I'm not (or I'm really not looking in the right spot).
> 
>> The LLVM-premerge-test project recently added presubmit on Linux.
>> Windows will hopefully follow soon (in beta right now I believe), and Mac afterward!  (Even though mac is lacking on the cloud provider availability)
>> 
>> 
>> . From the Harbormaster documentation [1]:
>> 
>>     You'll need to write a nontrivial amount of code to get this working today. In the future, Harbormaster will become more powerful and have more builtin support for interacting with build systems.
>> 
>> So while I appreciate all the efforts being made in this area, I still don't even know where to start if I want to setup pre-commit testing for libc++ today. However, the path is very clear with GitHub PRs and there are many options available.
>> 
>> Whenever I hear arguments of dividing the community, not being able to share infrastructure, the lack of Herald -- those all make a lot of sense to me and I think they're good arguments.
>> However, it is clear that folks who even think about these arguments are not paying the same cost for the lack of good pre-commit testing that I'm paying on a weekly basis, 
>> 
>> FYI that can read quite condescending... 
> 
> Sorry, that was really not my intent. The point I tried to make is that we have different priorities because we work on different projects, which have different requirements. So what may seem like just an annoyance to some might be a huge quality of life and productivity issue for someone else.
> 
> Louis
> 
>> 
>> 
>> because for me that outweighs everything else.
>> 
>> I don't know how to come to a decision here, all I know is that libc++ needs to get out of the status quo soon. And if the solution is that Harbormaster suddenly becomes usable without an unreasonable time investment from me, then I'm fine with that too. I'm not looking to switch to GitHub PRs for the sake of it, I'm looking to solve problems that are harming libc++ in the current system.
>> 
>> Cheers,
>> Louis
>> 
>> [1]: https://secure.phabricator.com/book/phabricator/article/harbormaster/ <https://secure.phabricator.com/book/phabricator/article/harbormaster/>
>> 
>> 
>>> On Feb 29, 2020, at 23:06, Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com>> wrote:
>>> 
>>> 
>>> 
>>> On Tue, Feb 25, 2020 at 4:19 AM Christian Kühnel via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> Hi Louis,
>>> 
>>> I think this is a good idea. We should start with some local experiments where people are willing to try it and figure out how well that works and what does not. Why not allow this for "not significant" changes? They are merged without review today, so we could do them with reviews (and automated tests) via pull requests instead.
>>> 
>>> I still feel this is only a recipe for confusion if "some" pull-requests are accepted on Github but not all. So -1 from me on this.
>>>  
>>> 
>>> @Mehdi
>>> - it does not favor to build common tooling: the recent work on enabling pre-submit CI tests on Phabricator is valuable and I'm looking forward to get this extended. But splitting the various ways of contributing to the repo just means more infrastructure to build to sustain this kind of efforts. (the infrastructure is easier built on GitHub by the way, but that is an argument in favor of migrating from Phab to GH for the full-project).
>>> 
>>> Oh I'm happy to add Github support as soon as someone switches on PRs. This is soooooo much easier to set up and maintain than the Phabricator integration. And we already have builds for the release branch (https://buildkite.com/llvm-project/llvm-release-builds <https://buildkite.com/llvm-project/llvm-release-builds>) anyway. So we could easily scale that up. And we can only get pre-merge testing on Phabricator to a certain point, as it's not triggering builds for ~50% of the code reviews.
>>> 
>>> @Chris Lattner
>>> Although I am one of the (many) people who would love to see us move from Phabricator to GitHub PRs, I think it is super important that we do the transition all at once to keep the LLVM community together.  I’m already concerned about the fragmentation the discourse server is causing, e.g. MLIR not using a -dev list.  I’d rather the community processes stay consistent.
>>> 
>>> Please allow me to disagree there. IMHO we're way too large and diverse of a project to do binary, overnight transitions.
>>> 
>>> You seem to be arguing the "how to transition" while there is no agreement on a transition happening in the first place.
>>>  
>>> We're also too large to follow a one-size-fits-all approach. If we agree,
>>> 
>>> I don't: we went with a monorepo because we believed that the one-size-fits-all would be more beneficial than splitting, both in terms of infrastructure, but also in terms of the practices of the community, etc.
>>> 
>>> Github PRs are the right glow, why take this step-by-step. We should have something like a list of important and supported use cases/interactions for the infrastructure. Then we could start working on them one-by-one and figure out if/how they could be implemented on Github and how we could do a smooth transition between these.
>>> 
>>> If Herald rules are important: Find a way to implement something similar for Github. Maybe there is even a market for such a tool.
>>> If transparency is the problem: Find a way to mirror PRs into Phabricator, so people can at least see them there. 
>>> We're not restricted to community contributions there. We can also pay someone to build the things we need.
>>> 
>>> One aspect here though is that we can pay someone to build the things we need in Phabricator, we can't change GitHub though.
>>> It was mentioned in the past that we should engage with GitHub and see if they would add the feature we're missing to their roadmap, if it hasn't been done I'd start there: building up this list of things that need to happens before we can agree towards a transition, and engaging with GitHub to have these. 
>>> 
>>> -- 
>>> Mehdi

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